1. Field of the Invention
This invention relates to the processing of reconstituted tobacco sheet, and, more particularly, to control of processes for applying hydrophobic coatings to such sheet and the hydrophobic coatings useful therefor, as well as tobacco sheets produced for such control and smoking products produced therefrom.
2. Description of the Prior Art
The application of hydrophobic coatings to the surface of reconstituted tobacco, in general, and particularly reconstituted cigar wrappers, is well known and is described, for example, in U.S. Pat. Nos. 3,185,161; 3,185,162; and 3,534,743. Briefly, the hydrophobic coating imparts to the reconstituted tobacco the desired property of a marked increase in resistance to moisture penetration which is reflected in less stickiness of the tobacco surface on exposure to the smoker's lips and less tendency to disintegrate in the mouth, particularly when the reconstituted tobacco products have been formulated with water-soluble or hydrophilic binders. For optimum performance, the hydrophobic coatings should be applied as discrete filsm within a relatively narrow range of application levels. If applied at a higher than needed rate, there may be adverse effects on automatic apparatus used in production thereof, undesirable slipping of smoking products in the smoker's mouth or changes in the smoking characteristics.
Thus, it is extremely important to be able to accurately control the levels at which the hydrophobic coatings are applied to tobacco sheet. However, such control is not easily achieved since the determination of coating levels necessarily implies the measurement of two parameters: (1) the coating efficiency, and (2) the coating quantity.
Coating efficiency tests are a measurement of the functionality (i.e., expected performance) of the coating and are, at best, relatable to coating quantity only in a semi-quantitative way. Coating quantity, on the other hand, may be an absolute measurement but may have no bearing at all to coating efficiency or functionality.
An example will serve to demonstrate the point: A reconstituted tobacco sheet with a dry sheet weight of 5 g./ft.sup.2 is sprinkled with 30 mg. of ethyl cellulose powder (a hydrophobic cellulose polymer) per ft.sup.2 just before the sheet is completely dry. The hydrophobicity of this "coated" sheet would be minimal compared to a similar sheet made by depositing 30 mg. of ethyl cellulose dissolved in 95% isopropanol per ft.sup.2 of dry sheet. In this latter case, the ethyl cellulose has been applied as a film and not as a powder. Therefore knowing the exact quantity of coating indicates little about the physical state of particle distribution which is directly relatable to the efficiency of the coating. Thus, the monitoring of effective coating levels on reconstituted tobacco products must include not only the absolute amount of the material applied but also the efficacy or efficiency of the hydrophobic barrier. This complicates the quantitative procedures since all procedures will relate to one or the other but not both factors involved in coating level determinations. For this reason, empirical methods must be used to establish the correlation between quantity and efficiency. For example, reconstituted products are prepared with known coating levels and these samples are then subjected to some form of efficiency test. After a correlation is established, the efficiency test is used to monitor the coating quantity off-line, either on the sheet or on the finished product.
Prior to the present invention there has been used a very subjective "lip-adhesion test" which on numerous repetitive trials has been found to be consistently reproducible and correlateable to coating quantity. This test has therefore been used as a quality control test to monitor coating quantity. Thus, experience over the years has indicated how to lay down an efficient coating or hydrophobic barrier on reconstituted tobacco products but the exact, quantitative correlation with coating quantity has only been semi-quantitative as determined by the "lip-adhesion" tests which only yield results in approximate ranges of actual coating quantity, i.e., as described by "low, medium or high", or "acceptable, borderline or unacceptable". Exact quantitation of the coating level has been missing as a production quality assurance test or product quality control test.
Various attempts have been made to evaluate coating efficiency on tobacco sheet material but, up to the present invention, these have not yielded the necessary quantitative parameters. Tests predicated on penetration of the tobacco sheet as by moisture, frictional resistance of the coated surface of the sheet, the time of travel of water droplets on an inclined plane of the coated tobacco sheet, and similar such tests, yield results which are not reproducible and, for the most part, lack quantitation. At best, the results obtained with such methods could be used only to distinguish between coated and uncoated sheets.
Coating quantity determinations also are subject to inconsistency. Usually, the best approach to quantification of a material is by direct analysis. However, with cellulose derivatives being preferred for coating tobacco sheet, attempts at analyses have been less than successful since they must be based on total carbohydrate content of the coated sheet from which is deducted the natural tobacco carbohydrate content, the remaining carbohydrate corresponding to the coating cellulose derivative. Unfortunately because of the low level of coating employed, the actual weight of coating carbohydrate is usually within the experimental error of such determination. Another method of quantifying the coating involves coating a fixed length of the stainless steel belt under the same conditions as coating tobacco sheet and removing the coating from the belt, drying and weighing. This would yield the expected amount of coating in units of weight per unit area. This approach suffered from a number of disadvantages, particularly lack of reproducibility and the lack of a quality control method for coating level on samples of sheet returned from the field.
Further attempts to quantify coating included the use of a dye, e.g. Du Pont Victoria Green, in the coating composition at a known concentration and visual observation of the distribution of the coating on the sheet during production. This is disadvantageous since production must be interrupted in order to perform the test and no quality control was provided in that field samples could not be monitored by this technique.
Tracer technology has been used in the field of analytical chemistry as an indirect means of quantifying a material difficult to quantify by direct methods. It is known that zinc oxide has been used as a tracer for on-line, non-destructive testing of coating weight in papermaking processes using X-ray fluorescent instrumentation as the means of detection (see F. P. Arendt and W. D. Gleson--TAPPI 58 96, 1975). According to this article, zinc oxide is added to the coating color at levels of 0.5-1.0%. Samples of reconstituted tobacco sheet containing zinc at levels of even as high as 3% were analyzed with an X-ray fluorescent instrument and it was found that the instrument was not sensitive enough to pick up such levels of zinc on the tobacco sheet, nor was it able to discriminate from the background zinc, i.e. zinc salts naturally occur in tobacco.
Tobacco being a very complex chemical and biochemical entity which is usual for natural materials of plant origin has presented substantial problems not only in direct analysis but also indirect analysis. Thus, attempts at the usual tracer techniques have been unsuccessful, mainly because of background interferences from the tobacco itself. Even the use of fluorescent tracer techniques which are extremely sensitive and often yield detection levels in the order of parts per billion were found to be inapplicable. For example, riboflavin (vitamin B2) is fluorescent in solution and it was expected that the presence of riboflavin in the coating solution should provide an easily analyzable tracer. However, it was found that background fluorescence of the tobacco made it impossible to accurately assess levels of the tracer.
It is known that tobacco contains various metallic constituents, including cobalt and zinc, at levels of about 0.2 to 7 ppm and about 24 to 53 ppm, respectively, as well as other metals as described by R. A. Nadkarni, Chemistry and Industry, page 693, September 1974. The same article reports a transfer of cobalt and zinc to smoke condensate of 4.2% and 2.7% respectively. Thus, low levels of cobalt and zinc are present in tobacco and the amounts of such metals carried over into smoke are even less.
As shown in U.S. Pat. Nos. 3,654,109 and 3,734,620, the use of atomic absorption spectroscopy is a well-known technique. U.S. Pat. No. 3,654,108 describes the use of atomic absorption spectroscopy in measuring the thickness of a metal coating deposited on a stationary substrate from the vapor phase. U.S. Pat. No. 3,734,620 shows the use of atomic absorption spectroscopy in measuring certain properties of materials such as temperature and density.
U.S. Pat. No. 3.016,460 describes the use of multistation radiation gauges to measure wet thickness.
The quantity per unit area of coatings on reconstituted tobacco sheet when prepared on commercial scale can vary appreciably. Such factors as wear of the doctor blade edge, bearings, and coating rollers, and loss of mechanical alignment can appreciably affect the weight per unit area of the coating. These variations can be responsible for uneven distribution of the coating across the width of tobacco sheet, as well as along the length of the sheet. Even employing as strict controls as heretofore possible, such variations normally amount to at least about 40%, and usually range from 40 to 80%, and sometimes 100%. Naturally, since coating of tobacco sheets is accomplished by relatively high speed belt application, there is an inherent tendency for somewhat larger quantities of coating material to deposit at the edges of the belt than at the center so that the distribution of coating across the width of the tobacco sheet is not even, but this inherent variation from center to edge is within tolerable limits. The present invention however does provide monitoring means for variations which exceed these inherent increments from the central regions to the outer periphery of the sheet. Control of such variations have heretofore been unsuccessful because of the lack of analytical procedures which permit accurate measurement of such coating weights within reasonable time periods.